CSE 681 Illumination and Phong Shading What is ?

 Physics tells us …  “We don’t see objects, we see light reflected off of objects”  “Light is a particle and a wave”

 The of light What is Color?

 Our visual system perceives the visible spectrum as color Primary Colors

 A color is a summation of primary colors  C(λ) = r(λ) + g(λ)G + b(λ)B

g(λ) r(λ) b(λ) Definitions

 Illumination  Transport of luminous  One or more light sources where light hits a surface either directly or indirectly

 Shading  The process of assigning a color to a pixel

 Illumination Models  Approximations to light transport  Goal: Physically correct models Illumination

 Light Sources (Emitters)  Positional, directional, area

 Surfaces (Reflectors)  Light source energy is cumulative (additive)  Absorb and emit light  Position and orientation  Smooth vs. Micro –structure (roughness) General Problem

 Robert Cook (Siggraph 1984)

“The intensity of reflected light at a point on a surface is an integral over the hemisphere above the surface of an illumination function L and a reflecance function f.”

Analytically Local vs. Global Illumination

 Global  Illuminate a point on a surface taking into account other surfaces  Shadows, , refraction,

 Local  Illuminate a point on a surface assuming it is the only point in the scene Phong Illumination Model

 Ambient  Gross (very cheap) approximation to indirect light hitting a surface after reflecting off of other surfaces  Absorbs this light and reflects surface color

 Diffuse  Light reflected off a surface equally in all directions after being absorbed by the surface (subsurface scattering) and then re-emitted  Reflects surface color

 Specular  Light reflected immediately off the surface  Light is not absorbed  Reflects light color The Phong Illumination Model

 Sum these three components: Illumination = ambient + +

Surface Ambient (Global Illumination)

 Radiosity  Calculate the propagation of light through the scene as it reflects off all surfaces  We see the reflection of all this indirect illumination  Computationally intensive to do correctly

 Ambient  A gross approximation to radiosity  Use a constant to represent the amount of indirect light  Yeah, it’s a hack! But global illumination is tough!

= Iambient kaIa Lambert’s Cosine Law

 Diffuse reflection is proportional to the amount of light that hits the surface per unit area Lambert’s Cosine Law

 Projected Area = cos(θ) = (N· L)

 Use max((N· L), 0) * colorobject

N θ L

Surface

dA Specular (Local Illumination)

 Light reflected at a reflection angle

n θ θ r l

Surface Specular Reflection

 The reflection of the light source on the object  Shiny/Glossy surfaces  Not a perfect mirror

Show up as Specular Highlights, i.e., bright spots Specular

 Cosine falloff about mirror reflection vector  View-dependent Reflectance Ray

N (L·N) N S S = (L·N)N - L R L · N L R = (L·N)N + (L·N)N - L

Surface R = 2 ( N.L ) N − L Cosine Falloff

 The angle between the ideal reflection direction R and the view direction V is α

cos(α) = V· R = L· VR N R θ θ eye α L V Surface Material Property

 (Cos(α))q = (V· R)q,  Power q = size of the lobe = how fast the specular component falls = glossiness of the surface

r Specular Reflection Coefficient Alternative Specular Calculation

 Half-way vector H bisects the angle between V and L  Compare H to N  Cheaper to compute than R

N H = (V + L)/|V + L| L α H q May use: (H · N) * Colorlight V

Surface Blinn-Torrance Variation

 Use halfway vector H between V and L

h n

β Eye l v Surface A Comparison

 Is the variation a good approximation?

 Difficult to distinguish visually – used in OpenGL!

Phong Blinn-Torrance Phone Illumination (Single Light)

 Terms

 Kd = diffuse coefficient, Ks = specular coefficient

 Let Ka = Kd, assume ambient term is diffusely reflected  I = light intensity

 Compute for each wavelength (r, g, b)

q C = (Kd Ia + N·L * Kd * Id) * colorobject+ (V· R) * Ks * Is * (1,1,1)

white light source Multiple Light Sources

 Simply add together the contribution from each light source equally

 Add in the effects of a light source iff the “face is a FRONT FACE” with respect to the light  The sign of N· L

 δi = 0 if light is behind face = 1 if light is in front of face

q C = Σiδi [ Kd * Iai + (N·Li* Kd * Idi) * cobj + (V· R) * K s* Isi * ci] Ambient/Diffuse/Specular

 Just ambient light:

 Diffuse + Specular and change Ambient

 Left: Sphere with just diffuse reflection  Right: Sphere with just specular reflection Ambient Only

 Cheap global illumination Ambient + Diffuse Ambient + Diffuse + Specular Phong Illumination